That is not correct. Any modifications to the project under the AGPL license need to also be under AGPL.
Anything that simply uses the projects APIs is not bound to the same license. If this were not the case, then companies using MongoDB would have to be releasing all their project code, including Google / Ebay / EA / Sage / anyone else listed here: https://www.mongodb.com/who-uses-mongodb
If all these high profile companies are OK with using an AGPL project without turning their projects open source, then I'm sure it's legally fine to do so in practise.
Anything that simply uses the projects APIs is not bound to the same license. If this were not the case, then companies using MongoDB would have to be releasing all their project code, including Google / Ebay / EA / Sage / anyone else listed here: https://www.mongodb.com/who-uses-mongodb
If all these high profile companies are OK with using an AGPL project without turning their projects open source, then I'm sure it's legally fine to do so in practise.